Light water reactors such as boiling-water reactors (BWR) and pressurized-water reactor (PWR) typically include fuel assemblies loaded into the reactor core as nuclear fuel. The fuel assembly includes a plurality of uranium-containing nuclear fuel rods (or simply, “fuel rods”) arrayed and supported with an upper tie plate and a lower tie plate.
Each nuclear fuel rod includes uranium fuel pellets charged into a fuel cladding tube about 4 meters long, and the both ends of the tube are sealed with end plugs. Traditionally, a zirconium alloy (zircalloy), which has a small thermal neutron absorption cross section and desirable corrosion resistance, has been used as material of the fuel cladding tube and the end plugs. This material has good neutron economy, and has been safely used in typical nuclear reactor environments.
In light water reactors using water as a coolant, generated heat from the uranium fuel raises the temperature inside the nuclear reactor, and a high-temperature water vapor generates in case of a loss-of-coolant accident (LOCA), a rare event where the coolant water fails to enter the nuclear reactor. In the event where the lack of the coolant (coolant water) exposes the fuel rods from coolant water, the temperature of the fuel rods well exceeds 1,000° C., and causes the zirconium alloy of the fuel cladding tube to chemically react with water vapor (the zirconium alloy is oxidized, and the water vapor is reduced) to generate hydrogen. Various safety measures are taken against a loss-of-coolant accident (LOCA), including, for example, an emergency core cooling system (ECCS). Such safety measures are not confined to system designs, but extend to the constituent materials of the reactor core.
For example, there are studies directed to using ceramic materials for fuel cladding tubes and end plugs, instead of using a zirconium alloy, which becomes a cause of hydrogen generation. Particularly, silicon carbide (SiC), which has desirable corrosion resistance, high heat thermal conductivity, and a small thermal neutron absorption cross section, has been a focus of active research and development as a promising material of fuel cladding tubes and end plugs. It is also expected that SiC greatly reduces hydrogen generation in case of a loss-of-coolant accident (LOCA), because the oxidation rate of SiC is two orders of magnitude smaller than the oxidation rate of a zirconium alloy in a high-temperature steam environment above 1,300° C.
For example, PTL 1 proposes a fuel cladding tube and end plugs configured from a SiC material. PTL 1 discloses a configuration in which a fuel cladding tube, and end plugs for sealing the both end portions of the fuel cladding tube are formed of a SiC fiber reinforced composite reinforced with silicon carbide continuous fibers, and in which the fuel cladding tube and the end plugs are directly joined to each other without interposing a dissimilar material, in at least a joint portion that comes into contact with the reactor coolant. This publication also describes a configuration in which the fuel cladding tube and the end plugs are directly joined to each other without interposing a dissimilar material on the side that comes into contact with the reactor coolant (the outer periphery surface side of the fuel cladding tube), and in which the side that does not come into contact with the reactor coolant (the inner periphery surface side of the fuel cladding tube) is joined by solid-state welding via a dissimilar material (a composite of titanium silicon carbide and titanium silicide, or silicon carbide containing aluminum and yttrium).